1. Field of the Invention
The present invention relates to a shift register, a display device having the shift register and a method of driving the shift register, and more particularly to a shift register capable of generating a scan signal for scanning a gate line of an active matrix thin film transistor liquid crystal display device (“AMTFT-LCD”), a display device having the shift register and a method of driving the shift register.
2. Description of the Related Art
Information processing devices have been developed to have various characteristics, such as various shapes, various functions, high speed, etc. The information processing devices generally process information using electric signals. A user of an information processing device may recognize information through a display device working as an interface tool between the user and the signals.
Such display device may be implemented with a cathode ray tube (“CRT”), a plasma display panel (“PDP”), an organic light emitting display (“OLED”), a liquid crystal display (“LCD”) device, etc. The LCD devices have various characteristics, such as lighter weight, smaller size, higher resolution and lower power consumption and are generally more eco-friendly than the CRT devices. The LCD device may display images having full-color.
In an LCD device, arrangement of liquid crystal molecules varies in response to an electric field applied thereto, so that light characteristics of the liquid crystal such as birefringence, luminance, diffusion, etc. are altered.
The LCD devices would be classified into a twisted nematic (“TN”) LCD, a super-twisted nematic (“STN”) LCD, a homogeneous LCD, etc. based on types of the liquid crystal molecular arrangements. The LCD devices are also classified into either an active matrix LCD having a switching member or a passive matrix LCD, based on the types of driving the liquid crystal. The active matrix LCD corresponds to the TN LCD, and the passive matrix LCD corresponds to the STN LCD.
The active matrix LCD devices use a thin film transistor (“TFT”) as a switching member, while the passive matrix LCD devices do not employ a switching member.
TFT LCD devices having TFTs as switching members are classified into either an amorphous-silicon (“a-Si”) TFT LCD or a poly-silicon (“poly-Si”) TFT LCD. The poly-Si TFT LCD devices generally have lower power consumption than the a-Si TFT LCD devices. However, the poly-Si TFT LCD devices have more complex manufacturing process than the a-Si TFT LCD devices. Therefore, the poly-Si TFT LCD devices are widely applied to small-screen display devices such as IMT-2000 cellular phones.
The a-Si TFT LCD is suitable for use in a large-screen display and has higher yield than the poly-Si TFT LCD. Therefore, the a-Si TFT LCD is widely applied to large-screen display devices, such as a notebook personal computer, an LCD monitor, a high definition television (HDTV) receiver set, etc.
FIG. 1 is a plan view showing a thin film transistor (TFT) substrate of a conventional poly-silicon liquid crystal display device (“poly-Si TFT LCD”).
Referring to FIG. 1, the poly-Si TFT LCD includes a glass substrate 10, an integrated printed circuit board (“PCB”) 20 and a film cable 18. A data drive circuit 12 and a gate drive circuit 14 are formed on the glass substrate 10. A terminal 16 and the glass substrate 10 are electrically connected to the integrated PCB 20 through the film cable 18. Therefore, the manufacturing cost of the poly-Si TFT LCD is decreased. In addition, the data and gate drive circuits 12 and 14 are directly formed on the glass substrate 10 so that power consumption of the poly-Si TFT LCD is decreased.
FIG. 2 is a plan view showing a TFT substrate of a conventional amorphous silicon liquid crystal display device.
Referring to FIG. 2, a data driver chip 34 is formed on a data flexible PCB 32 through a chip-on-film (“COF”) process. End portions of data lines of a pixel array are electrically connected to a data PCB 36 through the data flexible PCB 32. In addition, a gate driver chip 40 is formed on a gate flexible PCB 38 through the COF process. End portions of gate lines of the pixel array are electrically connected to a gate PCB 42 through the gate flexible PCB 38.
The gate drive circuit may be mounted on the data PCB so that the gate PCB is omitted. An LCD module having an integrated PCB, which does not have a gate PCB, is disclosed in Korean Patent Laid Open Publication No. 2000-66493.
The LCD module having the integrated PCB, however, includes the gate flexible PCB, although the LCD module does not have the gate PCB. Therefore, the number of the flexible PCBs in an a-Si TFT LCD is larger than that in a poly-Si TFT LCD, so that the a-Si TFT LCD has more complex outer lead bonding (“OLB”) process than the poly-Si TFT LCD, thereby increasing the manufacturing cost thereof.
A shift register of a gate drive circuit is disclosed in U.S. Pat. No. 5,517,542.
In this patent, the shift register for the gate drive circuit outputs signals using three clock signals. Each of stages of the shift register outputs an output signal using two clock signals of the three clock signals. Each of the stages is enabled using an output signal from a previous stage as an input signal. In addition, each of the stages is disabled using a feedback signal from a following stage.
To maintain a selected stage inactive, a stored charge in a capacitor is provided to a gate electrode of a pull down transistor. Therefore, when a threshold voltage of the pull down transistor exceeds the charged voltage of the capacitor as a result of over stress, the pull down transistor is erroneously turned off in the inactive state of the selected stage.
To overcome the problem, U.S. Pat. No. 5,517,542 employs a threshold voltage drift compensations circuit that increases a VDD voltage proportional to a threshold voltage drift to prevent a decrease in the conductivity of the pull down transistor.
However, a need still exists for a shift register that improves functional and operational reliability by effectively preventing or reducing deterioration of components of the shift register, and a display device employing such shift register.